The present disclosure relates generally to vehicle transmission systems and, more particularly, to a method and system for reducing tip-out oscillation effects in a clutch-to-clutch shift, electronically controlled automatic transmission.
In a vehicle equipped with an automatic transmission, the torque generated by the engine is transmitted to the vehicle mass through various mechanical components within the transmission. For example, FIG. 1a is a schematic diagram illustrating an existing transmission system 100, in which a pair of range clutches (Clutch 2 and Clutch 3) within a planetary gear device are engaged in order to provide a transmission input/output speed ratio having a value xe2x80x9crxe2x80x9d that is not equal to 1 (i.e., any gear ratio other than a direct drive ratio). In addition, Clutch 3 is shown grounded to the transmission case 102 through a freewheeler 104. In the case where the transmission system 100 is configured in a direct drive ratio, as illustrated in FIG. 1b, a different pair of range clutches (Clutch 2 and Clutch 4) within the planetary gear device are engaged to provide a direct drive ratio. Although there is a freewheeler 106 shown associated with Clutch 4, it is not grounded to the transmission case.
An accelerator pedal xe2x80x9ctip-outxe2x80x9d maneuver refers to the action of a driver releasing the pedal from a depressed position to a zero or near zero position (i.e., nearly or completely released). Subsequent to such a maneuver, the vehicle engine""s output torque is abruptly decreased. Through the use of freewheeler devices 104 (FIG. 1a) and 106 (FIG. 1b), the effects of an abrupt decrease in output torque (e.g., excitation/oscillation of the driveline) are prevented from being transmitted through the range clutches and exciting the axle 108. This type of transmission system is also referred to as a xe2x80x9cfreewheeler-to-clutchxe2x80x9d shift transmission.
On the other hand, the use of freewheeler devices increases the overall cost of the transmission systems. Accordingly, in a conventional xe2x80x9cclutch-to-clutchxe2x80x9d shift transmission system, the freewheeler devices are eliminated such that the gear train is fully locked if there is no gear shifting. However, in this configuration, an abrupt engine torque decrease can excite the driveline without a freewheeler connected to one or more of the range clutches. Although a driveline oscillation due to tip-out may be relatively innocuous where the torque converter 110 functions as a damper without locking up the torque converter clutch (Clutch 1), it can pose a more serious problem if the torque converter clutch is fully locked or, alternatively, if there is no torque converter in the system and Clutch 1 is instead used as a starting device. Therefore, it is also desirable to be able to reduce the effects of tip-out oscillation in a clutch-to-clutch shift transmission system that does not utilize freewheeler devices.
In an exemplary embodiment, a method for reducing oscillation in an automotive transmission system includes detecting a tip-out of an accelerator pedal position. Responsive to the detected tip-out, a pressure command is adjusted for a selected range clutch within the transmission system so as to reduce the pressure of the selected range clutch from a normal pressure level to a low pressure level, thereby producing a desired gear train slip in the transmission system. Then, the pressure command of the selected range clutch is re-adjusted after a determined period of time at the low pressure level so as to increase the pressure of the selected range clutch from the low pressure level, and thereby eliminating the gear train slip.
In another aspect, a system for reducing oscillation in an automotive transmission system includes a sensing means for detecting a tip-out of an accelerator pedal position. A controller adjusts a pressure command for a selected range clutch within the transmission system, in response to the detected tip-out, so as to reduce the pressure of the selected range clutch from a normal pressure level to a low pressure level, thereby producing a desired gear train slip in the transmission system. The controller re-adjusts the pressure command of the selected range clutch after a determined period of time at the low pressure level so as to increase the pressure of the selected range clutch from the low pressure level, and thereby eliminating the gear train slip.